Recently I tried creating a tabbed admin interface in Views - similar to the Content view where 3 tabs exist: Content, Commments, Media. It took a bit of futzing and reading, but ultimately found this:

1. create page with path "parent" and with content that you would normally have for default tab, don't set up menu settings
2. create page with path "parent/child1" and with content that you would normally have for default tab (SAME as "parent"), set it to be default tab and tell views Parent menu item: Already exists
3. create page with path "parent/child2" and with content you want, set it to be menu

Drupal has entered it's 8th cycle.  As such, Im regularly asked what I think the top usability issues are. Honestly, this is the first time I've put them down on paper, so this is a bit of a swag, but here it goes...

1. Connect the dots.  This would be my over-arching theme of Drupal 8.  D7 gave us a hint of this where links to configure and permissions were provided in context after enabling a module, but there's so much more to do.  One example of this is creating a menu item through the menu itself; you have to know the URL in which you want to link to.

Rob over at Mustardseed media created this create video on the context module. Check it out: